I'd be willing to bet it's original. Creating a simple loop like that is a lot easier than making a decent flash animation. Plus its a pay site full of original content so it stands to reason.
Hentai Key usually puts in original music fitting to the theme of these games. The one with that one chick from Naruto sounded very Ninja-foresty or whatever, and this sounds a little like bloodrayne, which contained a lot of goth-techno.
